1. Field of the Invention
The present invention relates in general to an electrically driven automotive vehicle, and more particularly to techniques for prolonging service life of an electrical energy storage device used on the vehicle.
2. Discussion of the Related Art
There is known an electrically driven or powered automotive vehicle of hybrid type including an internal combustion engine, an electric generator driven by the internal combustion engine to generate an electric energy, an electric energy storage device for storing the electric energy generated by the electric generator, and an electric motor operated by the electric energy supplied from the electric energy storage device. An example of the hybrid type electric vehicle is disclosed in JP-A-5-328526. In this hybrid type electric vehicle, the internal combustion engine is operated as needed, to drive the electric generator for generating an electric energy, so that a continuously running distance of the electric motor vehicle can be made sufficiently large, without having to use the electric energy storage device having a considerably large storage capacity. Further, the amount of fuel consumption by the engine and the amount of exhaust gas emissions from the engine during generation of the electric energy by the electric generator can be reduced by operating the engine at a speed at which the fuel economy is the highest, and by holding the engine off when the generation of the electric energy is not necessary. Owing to these practical advantages, the electric motor vehicle of the hybrid type has been drawing an attention of the industry.
The electric generator is usually operated by the engine to charge the electric energy storage device, for avoiding insufficiency of the electric energy stored in the storage device during running of the vehicle. For instance, the electric energy stored in the storage device is maintained within a range of 30-80% of its full storage capacity (nominal storage capacity), so that a given amount of electric energy is always left in the storage device.
However, some reports indicate that repeated charging of the electric energy storage device before it is fully discharged may cause a phenomenon of a gradual decrease in the storage capacity (maximum energy amount that can be stored in the storage device) before expiration of the expected service life of the storage device. To prevent this problem, it is known to completely or fully discharge the storage device before each charging of the storage device.